The present invention relates to a lens device and method of making same, a method of obtaining a stable focus servo signal, an optical pickup adopting the same, a method of discriminating discs having different thicknesses and a method of reproducing/recording information from/onto the discs.
An optical pickup records and reproduces information such as video or audio data onto/from recording media, e.g., discs (or disks). A disc has a structure that an information-recorded surface is formed on a substrate. For example, the substrate can be made of plastic or glass. In order to read or write information from a high-density disc, the diameter of the optical spot must be very small. To this end, the numerical aperture of an objective lens is generally made large and a light source having a shorter wavelength is used. However, in case of using the shorter wavelength light source, a tilt allowance of the disc with respect to optical axis is reduced. The thus-reduced disc tilt allowance can be increased by reducing the thickness of the disc.
Assuming that the tilt angle of the disc is 8, the magnitude of a coma aberration coefficient W.sub.31 can be obtained from: ##EQU1## where d and n represent the thickness and refractive index of the disc, respectively. As understood from the above relationship, the coma aberration coefficient is proportional to the cube of the numerical aperture (NA). Therefore, considering that the NA of the objective lens required for a conventional compact disc (CD) is 0.45 and that for a conventional digital video disc or digital versatile disc (DVD) is 0.6 (to accommodate the higher information density), the DVD has a coma aberration coefficient of about 2.34 times that of the CD having the same thickness for a given tilt angle. Thus, the maximum tilt allowance of the DVD is reduced to about half that of the conventional CD. In order to conform the maximum tilt allowance of the DVD to that of the CD, the thickness d of the DVD could be reduced.
However, such a thickness-reduced disc adopting a shorter wavelength (high density) light source, e.g., a DVD, cannot be used in a recording/reproducing apparatus such as a disc drive for the conventional CDs adopting a longer wavelength light source because a disc having an non-standard thickness is influenced by a spherical aberration to a degree corresponding to the difference in disc thickness from that of a normal disc. If the spherical aberration is extremely increased, the spot formed on the disc cannot have the light intensity needed for recording information, which prevents the information from being recorded precisely. Also, during reproduction of the information, the signal-to-noise (SIN) ratio is too low to reproduce the recorded information exactly.
Therefore, an optical pickup adopting a light source having a short wavelength, e.g., 650 nm, which is compatible for discs having different thicknesses, such as a CD or a DVD, is necessary.
For this purpose, research into apparatuses capable of recording/reproducing information on either of two disc types having different thicknesses with a single optical pickup device and adopting a shorter wavelength light source is under progress. Lens devices adopting a combination of a hologram lens and a refractive lens have been proposed in, for example, Japanese Patent Laid-Open Publication No. Hei 7-98431.
FIGS. 1 and 2 show the focusing of zero-order and first-order-diffracted light onto discs 3a and 3b having different thicknesses, respectively. In each figure, a hologram lens 1, provided with a pattern 11, and a refractive objective lens 2 are provided along the light path in front of discs 3a and 3b. The pattern 11 diffracts a light beam 4 from a light source (not shown) passing through hologram lens 1, to thereby separate the passing light into first-order-diffracted light 41 and zero-order light 40 each of which is focused to a different point on the optical axis with a different intensity by the objective lens 2. The two different focal points are the appropriate focus points on the thicker disc 3b and the thinner disc 3a, respectively and thus enable data read/write operations with respect to discs having different thicknesses.
However, in using such a lens system, the separation of the light into two beams (i.e., the zero order and first order light) by the hologram lens 1 lowers the utilizing efficiency of the actually used (reflected and partially twice diffracted, 1st order) light to about 15%. Also, during the read operation, since the information is riding on one of the beams while the other beam is carrying no information, the beam that is carrying no information is likely to be detected as noise. Moreover, the fabrication of such a hologram lens requires a high-precision process used in etching a fine hologram pattern, which increases manufacturing costs.
FIG. 3 is a schematic diagram of another conventional optical pickup device as disclosed in U.S. Pat. No. 5,281,797. This optical pick-up device includes a variable diaphragm 1a for varying the aperture diameter, so that data can be recorded onto a longer wavelength disc as well as a shorter wavelength disc, but with the discs having the same thickness, and information can be reproduced therefrom. The variable diaphragm 1a is installed between the objective lens 2 and a collimating lens 5. The variable diaphragm 1a controls a beam 4 emitted from a light source 9 and transmitted through a beam splitter 6, by appropriately adjusting the area of the beam passing region, i.e., the numerical aperture (NA). The diametral aperture of the variable diaphragm 1a is adjusted in accordance with the spot size required by the disc to be used and always passes the annular beam 4a of the central region but selectively passes or blocks the beam 4b of the peripheral region. In FIG. 3, a reference numeral 7 denotes a focusing lens and a reference numeral 8 denotes a photodetector.
In the optical device having the above configuration, if the variable diaphragm is formed by a mechanical diaphragm, its structural resonance characteristics change depending on the effective aperture of the diaphragm. The installation of the diaphragm onto an actuator for driving the objective lens becomes difficult in practice. To solve this problem, liquid crystals may be used for forming the diaphragm. This, however, greatly impedes the miniaturization of the system, deteriorates heat-resistance and endurance and increases manufacturing costs.
Another approach is disclosed in U.S. Pat. No. 5,496,995. As disclosed, a phase plate in placed in a light path of an objective lens. The phase plate creates first and second light sources of different phases such that the amplitudes of the lateral sides of a main lobe of an image of the first light source are cancelled by the amplitude of the main lobe of an image of the second light source by superimposition. In one embodiment, annular opaque rings separate grooves of different depths, the grooves providing the phase difference. A problem inherent to this approach is the need to carefully control the groove depth and light amplitudes, for example, to create the proper phase change and lobe cancellation.
Alternatively, a separate objective lens for each disc may be provided so that a specific objective lens is used for a specific disc. In this case, however, since a driving apparatus is needed for changing lenses, the configuration becomes complex and the manufacturing cost increases accordingly.